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=== Usage Situations===
* High capacitance lines
* High switching activities
=== Bus-Invert method ===
# Compute the [[Hamming Distance]] (the number of bits in which they differ) between the present bus value (also counting the present invert line) and the next data value.
# If the Hamming distance is larger than n/2, set invert = 1, and make the next bus value equal to the inverted next data value.
# Otherwise let invert = 0, and let the next bus value equal to the next data value.
# At the receiver side the contents of the bus must be conditionally inverted according to the invert line, unless the data is not stored encoded as it is (e.g., in a RAM). In any case, the vaue of invert must be transmitted over the bus (the method increases the number of lines from n to n+1).<ref name="r1" />
== Example ==
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== Performance Analysis ==
The Bus-Invert method generates a code that has the property that the maximum number of transitions per time-slot is reduced from n to n/2 and thus the peak power dissipation for I/O is reduced by half. From the coding theory point of view, the Bus-Invert code is a time-dependent Markovian code.
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* The invert line contributes itself with some transitions.
* The distribution of the Hamming distances for the next data values is not uniform.<ref>M. R. Stan and W. P. Burleson, “Bus-invert coding for low-power I/O,” IEEE Transactions On VLSI Systems, Vol.3, No.1, pp.49-58, 1995</ref>
== Partitioned Inversion Encoding ==
In order to decrease the average I/O power dissipation for wide buses the observation that the Bus-Invert method performs better for small n can be used to partition the bus into several narrower subbuses. Each of these subbuses can then be coded independently with its own invert signal. For example a 64-bit bus could be partitioned into eight 8-bit subbuses with a total of eight n order to decrease the average I/O power dissipation for wide buses the observation that the Bus-Invert method performs better for small n can be used to partition the bus into several narrower subbuses. Each of these subbuses can then be coded independently with its own invert signal. For example a 64-bit bus could be partitioned into eight 8-bit subbuses with a total of eight invert signals. Because of the assumption that the data to be transferred over the wide bus is random uniformly distributed, the statistics for the narrower subbuses will be independent and the sequence of data for each subbus will be random uniformly distributed. For example for a 64-bit bus partitioned into eight 8-bit subbuses the average number of transitions per time-slot will be 26.16 (8 times 3.27, the average for one 8-bit subbus) and the average number of transitions per bus-line per time-slot will be .41 (as for an 8-bit bus with one invert line). The maximum number of transitions is not improved by partitioning the bus and remains the same n/2. However, there is always an extra overhead of using more lines, but computationally, it has been found that the inversion bus encoding works well for most purposes.<ref name="r1">http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.89.2154&rep=rep1&type=pdf</ref>▼
▲In order to decrease the average I/O power dissipation for wide buses the observation that the Bus-Invert method performs better for small n can be used to partition the bus into several narrower subbuses. Each of these subbuses can then be coded independently with its own invert signal. For example a 64-bit bus could be partitioned into eight 8-bit subbuses with a total of eight n order to decrease the average I/O power dissipation for wide buses the observation that the Bus-Invert method performs better for small n can be used to partition the bus into several narrower subbuses. Each of these subbuses can then be coded independently with its own invert signal. For example a 64-bit bus could be partitioned into eight 8-bit subbuses with a total of eight invert signals. Because of the assumption that the data to be transferred over the wide bus is random uniformly distributed, the statistics for the narrower subbuses will be independent and the sequence of data for each subbus will be random uniformly distributed. For example for a 64-bit bus partitioned into eight 8-bit subbuses the average number of transitions per time-slot will be 26.16 (8 times 3.27, the average for one 8-bit subbus) and the average number of transitions per bus-line per time-slot will be .41 (as for an 8-bit bus with one invert line). The maximum number of transitions is not improved by partitioning the bus and remains the same n/2. However, there is always an extra overhead of using more lines, but computationally, it has been found that the inversion bus encoding works well for most purposes.
== See also ==
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